The preferred embodiments are in the field of amplifier gain stages and are more specifically directed to a single-ended gain stage such as for use with a disk drive and related data or computing system.
A gain stage is known for providing an output signal that may used for various purposes, where one example is a single-ended output, that is, only one voltage output based on a differential input. Both as context and as an example for a preferred embodiment, such a single-ended output of a gain stage may be used in the control loop for a disk drive so as to impart energy to the motor that positions the read/write head(s) of the drive. In the computing art, such a motor is sometimes referred to as a voice coil motor (“VCM”) and the control loop is typically a part of an integrated circuit, sometimes referred to as a servomotor (or, abbreviated “servo”) control. Particularly, the servo control loop provides a feedback based on the sensed current through the VCM, where that current may provide an indication of one or both of the VCM position or speed and it is therefore “sensed” by a gain stage that is often referred to as a sense amplifier.
The sense amplifier described above may be implemented in various forms, but typically the amplifying circuitry (e.g., transistors) that implement it are susceptible to common mode rejection. Specifically, and due at least in part to a lack of ideal characteristics of its differential amplifying circuitry, the sense amplifier not only has a gain (e.g., Ad) that amplifies its intended differential input signal, it also has a gain (e.g., Ac) that amplifies any signal variations in the signal that is common to both of its differential inputs. In other words, there is a phenomenon of amplifying the inputs signals that are common to both inputs, and the ability of an amplifier system to reject those common signals is referred to as a common mode rejection ratio (“CMRR”). Further, therefore, in certain applications it is desirable to reduce the CMRR, and since CMRR is typically defined in terms of decibels as
      CMRR    =          20      ⁢              log        10            ⁢                                            A            d                                A            c                                        ,then a change in 20 dB corresponds to a change in the ratio of Ad to As of 10. Thus, an improvement in CMRR is represented by an increase in the absolute value of the CMRR; for example, for a system with a 60 dB CMRR, it may be desirable to improve the CMRR of the system by increasing that ratio to 80 dB.
The sense amplifier described above also is typically implemented in one of various known differential input single output configurations wherein resistor combinations are used to establish the overall gain of the sense amplifier configuration. In these configurations, it is also known in the art that a mismatch in the resistance of the resistors used in the configuration, or in the ratio of such resistance, also affects the common mode introduced into the input of the amplifier (and, hence, affecting the intended output signal).
In view of the preceding, the prior art endeavors to adjust the resistance of the resistors used in the gain stage configuration so as to reduce the CMRR of the stage. In this regard, and as also discussed later, the matching of resistance for certain of those resistors must be increased to realize a corresponding improvement in CMRR. For example, to improve CMRR by 20 dB, the matching level of these certain resistors needs to be improved by a factor of 10. Typically, resistor matching level is usually proportional to 1/√{square root over (WL)}, where W and L are respectively the width and the length of the resistor as formed as a semiconductor (e.g., polysilicon) region in an integrated circuit. Therefore, to achieve a denominator of 10 in the value of 1√{square root over (WL)} the product of WL must equal 100. In other words, either or both the width and the length of the resistor must be vastly increased, which results in a greatly increased area for the entire sense amplifier configuration. As known in the art, such a large increase in area is undesirable for many reasons. For example, an increase in the circuit affects the cost and size of the circuit as well as the device, such as a magnetic disk drive, in which the circuit is used.
Given the preceding, the preferred embodiments seek to improve upon the prior art, as demonstrated below. And, by improving upon the prior art, improved CMRR may be achieved in a gain stage without a large increase in the gain stage circuit size, thereby facilitating a lower cost and device size for the circuit and device in which the circuit is used. In this regard, magnetic disk drives are used over a wide range of electronic systems, including large-scale network servers, desktop computers and workstations, portable computers (e.g., laptops), and now even in modern handheld devices such as portable digital audio players. Accordingly, a decrease in size positively affects cost in such systems, and it also permits even further uses the use of drives in smaller equipment, including the above-mentioned audio players and with likely additional applications in the future.